Cosine Modulated Filter Bank, CMFB modulation for digital data transmission offers high spectral containment, low overhead, and high flexibility in spectrum usage. However, it is sensitive to signal distortions introduced by the transmission channel.
The CMFB modulation is an improved version of Orthogonal Frequency Division Multiplex, OFDM modulation. Both modulation types support flexible spectrum usage. The advantages of CMFB over OFDM are (i) the better contained transmit spectrum, obtained by additional filtering, and (ii) the higher efficiency, since the cyclic prefix overhead of OFDM can be omitted. However, CMFB implementation has higher computational complexity, and CMFB reception is highly sensitive to signal distortions introduced by the communication channel. Hence, the channel equalization is needed at the CMFB receiver to mitigate the effect of signal distortion on transmission performance.
A simple CMFB equalizer uses the same principle as the well-known OFDM equalizer, i.e. known training signals are inserted by the transmitter, e.g. as packet preamble that are OFDM symbols with known modulation on all subcarriers, or as pilot tones that are known modulation on known subcarriers. The receiver uses the corresponding received signals to estimate the channel transfer function C(f) in the frequency domain, i.e. after demodulation. The equalizer is then simply W(f)=1/C (f), i.e. a simple complex one-tap multiplication per sub-channel f, see Behrouz Farhang-Boroujeny, “Multicarrier Modulation with blind Detection Capability using Cosine Modulated Filter Banks,” IEEE Transactions on Communications, Vol. 51, No. 12, December 2003, pp. 2057-2070. This method suffices however only for transmission channels with short impulse responses and negligible inter-symbol interference, ISI. Also, in contrast to the OFDM case, the frequency domain channel estimation in CMFB is affected by the inherent inter-channel interference, ICI and thus converges only slowly.
Ihalainen et al., “Channel Equalization for Multi-Antenna FBMC/OQAM Receivers” addresses to the problem of channel equalization in filter bank multicarrier FBMC transmission based on the offset quadrature-amplitude modulation OQAM subcarrier modulation. Finite impulse response FIR per-subchannel equalizers are derived based on the frequency sampling FS approach, both for the single-input multiple-output SIMO receive diversity and the multiple-input multiple-output MIMO spatially multiplexed FBMC/OQAM systems. The FS design consists of computing the equalizer in the frequency domain at a number of frequency points within a subchannel bandwidth, and based on this, the coefficients of subcarrier-wise equalizers are derived. The paper evaluates the error rate performance and computational complexity of the proposed scheme for both antenna configurations and compare them with the SIMO/MIMO OFDM equalizers. The results obtained confirm the effectiveness of the proposed technique with channels that exhibit significant frequency selectivity at the subchannel level and show a performance comparable with the optimum minimum mean-square-error equalizer, despite a significantly lower computational complexity.